(1) Field of the Invention
The invention relates to the fabrication of Printed Circuit Boards (PCB's) that are used with integrated circuit devices, and more particularly, to a method of creating a structure for the mounting of semiconductor die.
(2) Description of the Prior Art
Printed Circuit Boards (PCB's) are being used extensively in the creation of large semiconductor functional units. The PCB serves a number of different functions when used to mount semiconductor devices such as providing mechanical or structural support for the semiconductor devices, the ability to significantly increase the number of Input/Output (I/O) terminals, the ability to reduce thermal constraints that otherwise would be imposed on the semiconductor device. To enable the mounting of semiconductor devices on the surface of a PCB, different device packages have been developed. Among these different packages, the Quad Flat Package (QFP) and the Ball Grid Array (BGA) package are frequently used.
Quad Flat Packages (QFP) have in the past been used to create surface mounted high pin count integrated packages with various pin configurations. The electrical connections with these packages are typically established by closely spaced leads that are distributed along the four edges of the flat package. This limits the usefulness of the QFP since a high Input/Output (I/O) count cannot be accommodated in this manner. To address this problem, the Ball Grid Array (BGA) package has been created whereby the I/O points for the package are distributed not only around the periphery of the package but over the complete bottom of the package. The BGA package can therefore support more I/O points making this a more desirable package for high circuit density with high I/O count. The BGA contact points are solder balls that in addition facilitate the process of flow soldering of the package onto a printed circuit board. The solder balls can be mounted in an array configuration and can use 40, 50 and 60 mil spacings in a regular or staggered pattern.
Increased I/O count combined with increased requirements for high performance IC's has led to the development of Flip Chip packages. A flip chip is a semiconductor chip that has a pattern or arrays of terminals spaced around an active surface of the flip chip for face down mounting of the flip chip to a substrate. Flip chip technology fabricates bumps (typically Pb/Sn solder) on Al pads on the chip and interconnects the bumps directly to the package media, which are usually ceramic or plastic based. The flip-chip is bonded face down to the package through the shortest paths. These technologies can be applied not only to single-chip packaging, but also to higher or integrated levels of packaging in which the packages are larger, and to more sophisticated package media that accommodate several chips to form larger functional units. Flip chip attachment consists of attaching a flip chip to a printed circuit board or other substrate. The flip chip is bonded to the printed circuit board by refluxing the solder balls. The solder balls may also be replaced with a conductive polymer.
The flip-chip technique, using an area array, has the advantage of achieving the highest density of interconnection to the device and a very low inductance interconnection to the package. However, pre-testability, post-bonding visual inspection, and Temperature Coefficient of Expansion (TCE) matching to avoid solder bump fatigue are still challenges. In mounting several packages together, such as surface mounting a ceramic package to a plastic board, the TCE mismatch can cause a large thermal stress on the solder lead joints that can lead to joint breakage caused by solder fatigue from temperature cycling operations.
In packaging ball grid array and flip-chip semiconductor devices, these devices are mounted on the surface of a package substrate; the plastic package substrate typically is an extension of a Printed Circuit Board made using the more advanced PWB technology. The contact points of the BGA/flip-chip make contact with contact points in the top surface of the substrate, the substrate re-distributes (fan-out) the BGA/flip-chip contact points. The lower surface of the substrate has the contact points (balls) that are connected to the surrounding circuitry and that form the interface between the BGA/flip-chip contact balls and this surrounding circuitry. The original contact balls of the BGA/flip chip packages are encased in a molding material (for instance epoxy) for protection of these balls. The molding is encased between the lower surface of the BGA/flip-chip device and the upper surface of the substrate. This molding is referred to as underfill since it is filled in under the original BGA/flip-chip device.
In PCB manufacturing, multiple layers of printed circuits are created inside the printed circuit board. These layers are superimposed and are electrically isolated from each other. The printed circuits that make up the various layers of the PCB establish the electrical interconnections between the semiconductor devices and the surrounding circuitry.
Prior Art substrate packaging uses ceramic and plastic Ball Grid Array (BGA) packaging. Ceramic substrate packaging is expensive and has proven to limit the performance of the overall package. Recent years have seen the emergence of plastic substrate BGA packaging; this type of packaging has become the mainstream design and is frequently used in high volume BGA package fabrication. The plastic substrate BGA package performs satisfactorily when used for low-density flip chip Integrated Circuits (IC's). If the number of pins emanating from the IC is high, that is in excess of 350 pins, or if the number of pins coming from the IC is less than 350 but the required overall package size is small (resulting in a solder ball pitch of less than 1.27 mm.), the plastic BGA structure becomes complicated and expensive. This can be improved by using the multi-layer structure used to create the plastic BGA package. This multi-layer structure deposited on both sides of a core board, one layer at a time for the plastic BGA interconnect package, is referred to as the Build Up Multilayer or BUM approach and results in a line density within the package of typically 2-3 mil or 50u-75u range. This line density is not sufficiently high for realizing the fan out from the chip I/O to the solder balls on the package within a single layer. This leads to the multi-layer approach. The multi-layer approach brings with it the use of relatively thick (50u-75u) dielectric layers, these layers have a TCE (Thermal Coefficient of Expansion) that is considerably higher than the TCE of the laminate board on which the plastic BGA package is mounted. To counteract this difference in TCE's the BUM layers must be (thermally and mechanically) balanced on the other side (the side of the board that does not usually require an interconnect density provided by the BUM layers) of the laminate board. This latter requirement results in the use of additional material and processing steps to apply these materials, increasing the cost of the BGA package and creating a yield detractor.
Another approach is the use of a flexible film as the starting material. A polymer film, such as the polyimide film or an epoxy based film of 2 to 3 mil thick with or without a copper layer attached to it, is processed by metalization and patterning on one or both sides. A completed two metal layer film, described as a layer pair, can be used as a packaging substrate material. Subsequent dielectric and copper layers can be build up on the processed first metal layer, such as the RCC (Resin Coated Copper) approach. Alternatively, two or more layer pairs can be bonded together to make a multilayer structure. The advantage of this approach is that it uses a minimum amount of material. However, because of the lack of stability of the film, the line and space density is limited to that of the BUM structure described herein, which is not sufficiently high for the high density packaging that is used to achieve a low cost substrate, having only a few interconnect layers.
Other Prior Art applications use thin film interconnect layers for flip chip or wire bond packaging substrates. These applications start with a laminate substrate onto which the thin film layers are deposited. For these applications, the laminate substrate is used as a base carrier substrate and provides the mechanical support. Plated Through Holes (PTH) are mechanically drilled through the laminate substrate and are used to establish connections to the backside of the substrate for solder ball attach and electrical contacts. By using thin films, high wire density and very thin dielectric layers can be realized. This approach also does not, unlike the BUM approach, require to counter-balance thick layers of dielectric in order to establish dimensional stability. A disadvantage of the laminate substrate is that the process of mechanically drilling holes through the laminate substrate is time-consuming thereby adding cost to the process. Further, the planarity of the laminate substrate does not meet planarity requirements for the deposition of thin films. Improved planarity for the surface of the laminate substrate is established by depositing dielectrics and metal layers on the initial surface of the laminate structure, steps that again add to the processing cost of the BGA structure. Since the laminate substrate is composed using organic materials, the substrate is not dimensionally stable resulting in warpage and dimensional variations during high temperature processing and wet chemical interactions. This results in additional processing complications and costs.
This invention teaches a novel process and structure for creating packaging substrates that are used for wire bonded and flip chip semiconductor devices. As such, the process and package of the invention are similar to previous high-density flip chip BGA packages. The term BGA of the invention refers to the ball grid array that is connected to for instance a Printed Circuit Board but where the contact balls of this array are connected to a (wire bonded or flip chip) IC device via a substrate that is created by the process of the invention.
U.S. Pat. No. 5,509,553 (Hunter, Jr. et al.) shows a (3) metal layer process (DEMR) (see FIG. 5A) that appears to comprise a) sputter plating base b) plating metal (semi-additive plating), see col. 2.
U.S. Pat. No. 5,830,563 (Shimoto et al.) discloses a laminate substrate with thin films deposited thereon.
U.S. Pat. No. 5,837,427 (Hwang et al.) shows a (4) BUM process for a PCB.
U.S. Pat. No. 5,724,232 (Bhatt et al.) shows a package with a (1) metal substrate.
U.S. Pat. No. 5,525,834 (Fischer et al.) shows a package having a Cu substrate, thin dielectric layers (1-25 um thick) and thin dielectric layers (12 to 75 um), see col. 7 and 8.
U.S. Pat. No. 5,877,551 (Tostado et al.) discloses a package having a metal substrate with (2) dielectric layers formed of polymers, epoxy (3 to 100 um), see col. 4.
U.S. Pat. No. 5,485,038 (Licari et al.) teaches a package using a photo-imagable epoxy dielectric layer.